Polyvinylidene fluoride (PVDF) based polymers are melt-processable fluoropolymer resins that are used to form polymer structures by many processes, such as extrusion, injection molding, fiber spinning, extrusion blow molding and blown film. They are also used as polymer processing aids because of their low surface energies and phase behaviors.
PVDF resin exhibits a poor elasticity in the melt especially for low molecular weight due to its linear chain architecture and relatively narrow molecular weight distribution (polydispersity ˜2 to 3). Typically, increasing the molecular weight and introducing co-monomers with different chemical composition overcome this problem. However, these approaches either make the polymer more difficult to process or change the bulk properties of the resin due to the presence of the co-monomer. An alternative to enhance the elasticity and the melt strength of this polymer without changing its chemical composition is by exposure to controlled amounts of radiation.
Open celled foams as described in US 2003/0077455, provide low-density cellular thermoplastic materials that are useful in applications such as cushioning in autos and planes, filtration media, and for shock absorption. Foams made according to the said patent are generally thermoplastic fluoropolymers with low crystallinity, as a result, they exhibit poor solvent resistance and poor compression set, and deformed easily at elevated temperature.
High levels of radiation have been used to graft and crosslink several different polymers, including fluoropolymers. There is no mention of the use of low-level radiation to improve polymer properties, especially on low crystalline polymers.
There is a need for higher melt strength fluoropolymers in many applications. There is also a need for a method of producing solvent resistance fluoropolymer foams having wide range of densities with good mechanical integrity at elevated temperature.
It has now been found that the use of low levels of ionized irradiation in a controlled manner can be used to increase the melt strength and elasticity of a fluoropolymer without changing the chemical composition, and also to produce a dense, flexible material by the thermoforming and crosslinking of foams.